This invention relates primarily to power switching devices such as solid state relays and power bridge circuits which have a relatively high current power switching circuit under the control of an isolated relatively low current and voltage control circuit.
Typical power switching devices are characterized as hybrid circuits in which conductive films and usually resistance elements are deposited on a flat ceramic substrate plate and a lead frame is adhered to the film conductors. Active circuit components such as triacs and silicon controlled rectifiers are mounted on strips which comprise the lead frame. A typical power switching device of this character may be seen in U.S. Pat. No. 3,958,075 which is owned by the assignee of this invention. As is known, a thin film soldering material is applied to the interfaces between components on the substrate before assembly and, after assembly, the substrate with the components mounted thereon is heated to cause the solder to reflow and bond the parts together. Usually the outside surface of the ceramic substrate is bonded at the same time to a rather massive metal plate which has bolt holes which enable pressing the assembly against the surface of a heat sink. Metallization and solder layers in combination with the base plate result in a large number of interfaces or junctions which impede transfer of heat from the substrate to the heat sink. Every junction produces a significant reduction in heat transfer ability which, in the last analysis, requires the manufacturer to specify lower current ratings for the devices than would be necessary if better heat dissipation could be obtained.
Continuing with the description of the prior art structure, after the metal base plate and substrate are joined by reflowing, the device is placed in an appropriate mold of an injection molding machine and it is encapsulated, often in epoxy resin, to form a unitary device from which terminals for making external connections project.
When a user mounts a device of this type on a heat sink, it is usually done by clamping the base plate to the heat sink with machine bolts or screws. The base plate is pulled down tightly with the bolts in an effort to cause the base plate and heat sink surface areas to be in overall contact. Higher compressive force, of course, improves heat exchange between the bottom of the base plate and the surface of the heat sink. In this prior type of device, however, regardless of the compressive force developed at the junction or interface of the base plate bottom and the heat sink, no additional compressive force is transmitted back to the other junctions or interfaces so that their heat transfer ability is not enhanced beyond that which is an inherent result of the method of manufacture.